Field of the Invention
This invention relates generally to a system and method for validating an estimation of hydrogen gas in an anode of a fuel stack and, more particularly, to a system and method for validating an estimation of hydrogen gas in an anode of a fuel cell stack and correcting the estimation if an error is identified, where the method includes comparing a measurement from a hydrogen gas virtual sensor to the estimation of the hydrogen gas that is determined using a gas concentration estimation model.
Discussion of the Related Art
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell type for vehicles, and generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer, where the catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. A fuel cell stack typically includes a series of flow field or bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Many fuel cell system control algorithms require knowing the concentration of hydrogen gas in the anode sub-system of the fuel cell system for various purposes, such as maintaining fuel cell stack stability, promoting a healthy start-up/shutdown sequence of the system, and initiating a hydrogen gas injection event to maintain hydrogen in the anode side during system off-time. It is possible to provide a gas concentration sensor at a strategic location in the fuel cell system, such as the output of the anode, to measure the concentration of the particular gas, such as hydrogen. However, in order for these types of sensors to provide an accurate estimation of the gas in the hot and wet environment of a fuel cell system, the sensors are very expensive, and still are not fully reliable, thus rendering them ineffective for automotive fuel cell system applications.
The MEAs in the fuel cells are permeable and thus allow nitrogen in the air from the cathode side of the stack to permeate through and collect in the anode side of the stack, often referred to as nitrogen cross-over. Even though the anode side pressure may be slightly higher than the cathode side pressure, cathode side partial pressures will cause air to permeate through the membrane. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases above a certain percentage, such as 50%, fuel cells in the stack may become starved of hydrogen. If a fuel cell becomes hydrogen starved, the fuel cell stack will fail to produce adequate electrical power and may suffer damage to the electrodes in the fuel cell stack. Thus, it is known in the art to provide a bleed valve in the anode exhaust gas output line of the fuel cell stack to remove nitrogen from the anode side of the stack. The fuel cell system control algorithms will identify a desirable minimum hydrogen gas concentration in the anode, and cause the bleed valve to open when the gas concentration falls below that threshold, where the threshold is based on stack stability.
It is known in the art to estimate the molar fraction of nitrogen and other gases in the anode side of a fuel cell stack using a model to determine when to perform the bleed of the anode side or anode sub-system. For example, gas concentration estimation (GCE) models are known for estimating hydrogen, nitrogen, oxygen, water vapor, etc. in various volumes of a fuel cell system, such as the anode flow-field, anode plumbing, cathode flow-field, cathode header and plumbing, etc. U.S. Pat. No. 8,195,407 issued Jun. 5, 2012 to Salvador et al., assigned to the assignee of this invention and herein incorporated by reference, describes one exemplary GCE model for this purpose.
It has been shown that these types of GCE models are susceptible to a number of operating conditions of the fuel cell system that can cause the GCE model to provide a relatively inaccurate estimation of the particular gas. Additionally, component failures and degradation of the components in the fuel cell system, such as the fuel cell membrane, may also cause errors in the model estimation. If the anode nitrogen molar fraction estimation is significantly higher than the actual nitrogen molar fraction, the fuel cell system will vent or bleed more anode gas than is necessary, i.e., will waste hydrogen fuel. If the anode nitrogen molar fraction estimation is significantly lower than the actual nitrogen molar fraction, the system will not vent enough anode gas and may starve the fuel cells of reactants, which may damage the electrodes in the fuel cell stack. In addition, current fuel cell system processes do not allow for correction of the hydrogen gas estimation if it is determined to be inaccurate.